An implantable medical device (IMD) can be used to monitor one or more tissue sites or to provide electrostimulation to one or more tissue sites such as to elicit or inhibit a contractile response in muscle tissue, or to provide one or more other forms of electrostimulation (e.g., a neural stimulation, such as directed to a vagal site, such as for pain management, to provide a therapy for one or more heart arrhythmias, or for one or more other therapeutic or diagnostic uses). For example, spontaneous intrinsic depolarizations can be generated by a heart itself, while evoked intrinsic depolarizations can be the result of an electrostimulation pulse such as a pacing pulse. Depolarization of heart tissue can cause it to contract. After contraction, while a heart chamber is expanding to fill with blood, repolarization of the heart tissue can occur allowing subsequent depolarization. An implantable medical device such as a cardiac rhythm management device (CRMD) can deliver electrostimulation pulses to regulate such heart tissue contraction. Examples of a cardiac rhythm management device can include, among other things, a pacemaker, a defibrillator, a cardioverter, a cardiac resynchronization device, or one or more other devices combining some or all such capabilities or one or more other capabilities.
Also, cardiac rate, contractility and excitability can be modulated through central nervous system mediated reflex pathways, which can include portions of the sympathetic and parasympathetic components of the autonomic nervous system. For example, baroreceptors and chemoreceptors in the heart, great vessels, and lungs can transmit cardiac activity information through parasympathetic and sympathetic afferent nervous fibers to the central nervous system. Increase of sympathetic afferent activity can trigger reflex sympathetic activation, parasympathetic inhibition, blood vessel constriction, and tachycardia. In contrast, parasympathetic activation can result in bradycardia, blood vessel dilation, and inhibition of vasopressin release.
The balance between the sympathetic and parasympathetic components of the autonomic nervous system can be referred to as the autonomic tone. Decreased parasympathetic or vagal tone can be a factor that can contribute to or cause various cardiac tachyarrhythmias. Such tachyarrhythmias can include atrial fibrillation and ventricular tachycardia, for example. In an example, an implantable medical device can deliver neural stimulation, such as to elicit the reflex response of parasympathetic activation or of sympathetic inhibition. In certain examples, the neural stimulation can include one or more of magnetic, electrical, optical or acoustic stimulation of neural targets.